Certain types of nonvolatile memory devices store data using a variable resistance material. Examples of such devices include phase change random access memory (PRAM), resistive RAM (RRAM), and magnetic RAM (MRAM).
A PRAM, for instance, comprises a plurality of phase change memory cells each comprising a phase change material that can assume a high resistance state or a low resistance state based on its lattice structure. The lattice structure can be changed by controlling the temperature of the phase change material with electrical current. The high resistance state, also referred to as a crystalline state, corresponds to a crystalline lattice structure, and the low resistance state, also referred to an amorphous state, corresponds to an amorphous lattice structure. The crystalline state is typically defined to represent “set data” or a logical “0”, and the amorphous state is typically defined to represent “reset data” or a logical “1”.
One shortcoming of conventional PRAM devices is that the resistance value associated with a memory cell storing “set data” may drift over time. This phenomenon is referred to as a set resistance drift. Another shortcoming is that the resistance value immediately after reset data is written may be smaller than a target value. Then, after the reset data is written and a predetermined reset stabilization time (tWTR) elapses, the resistance value reaches the target value. That is, according to a read time point, the resistance value of the set data/reset data may be changed.